1. Technical Field
The present disclosure relate to an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof.
2. Description of the Related Art
In electrophotographic image forming apparatuses, an electrostatic latent image, which is obtained by forming optical image data on an image carrier (e.g., a photoconductor) that is uniformly charged in advance, is rendered visible with toner from a development device. An image is formed on a recording medium by transferring the visible image directly or indirectly onto the recording medium (e.g., transfer sheet) via an intermediate transfer member and fixing the image thereon.
In a thus-configured image forming apparatus, a constant current control method to control a direct current (DC) transfer bias applied to a transfer member using a direct current (DC) power source is widely used. In constant current control, an output voltage from a bias application circuit is detected by a detection circuit provided to the bias application circuit, and a resistance of a transfer unit roller (i.e., resistance including the image carrier and the recording medium) is calculated based on the detected output voltage to determine a transfer current value.
However, at present, various types of recording media, for example, waved laser-like paper having premium accent or Japanese paper, are widely sold. In these papers, in order to create luxurious mode, surfaces of the papers have asperities with embossed effect. The toner in a concave portion of the paper is hardly transferred, compared to a convex portion thereof. More particularly, when the toner is transferred on the recording medium having large asperity, the toner cannot be transferred on the concave portion sufficiently, which may generate image failure in which toner image is partly absent.
In order to solve the transfer failure in the concave portion of the recording media, the related art discloses an approach in which a superimposed bias in which an alternating current (AC) voltage is superimposed on a direct current (DC) voltage is applied, and as a result, transfer efficiency is improved and image failure alleviated. In this configuration, in order to switch between the DC transfer mode and the superimposed transfer mode, the image forming apparatus has a DC power source to apply a DC transfer bias and a superimposed power source (AC+DC power source) to apply the superimposed bias.
In addition, the DC power source can be used to detect the resistance of the transfer portion to correct the value of an applied transfer bias.
However, with a superimposed bias, the resistance cannot be accurately calculated due to fluctuations in the alternating-current voltage over time.